Per-channel optical amplification using saturation mode

ABSTRACT

An optical communication system includes a plurality of optical channels, each of which passes a single optical wavelength signal. Each of the plurality of optical channels includes an optical amplifier which is controlled to operate at a predetermined output power level independent of channel wavelength and input power level by operating each optical amplifier in a saturation mode. Pumping power for operating each optical amplifier in the saturation mode is supplied from shared optical pumps or a plurality of one per channel optical pumps.

This application is a continuation U.S. application Ser. No. 09/461,052,filed Dec. 15, 1999 now U.S. Pat. No. 6,735,394.

FIELD OF THE INVENTION

The invention is in the field of optical telecommunications, and moreparticularly, pertains to an optical communication system in whichindividual channel output power levels are equalized independent ofchannel wavelength and input power level.

BACKGROUND OF THE INVENTION

In Wavelength Division Multiplexed (WDM) optical links it is difficultto assure that signals arriving at each channel's photodetector have apower level that is within the receiver's dynamic range. Even for simplepoint-to-point links, flattening filters are used in the Erbium DopedFiber Amplifiers (EDFA's), MUX/DEMUX components' profiles of attenuationvs. wavelength must be trimmed, and the system must be carefullymonitored to ensure that large inter-channel differences in concatenatedconnector and splice losses are not accumulated.

Typically, all WDM channels are amplified in a single amplifier, withthe single amplifier being optimized for gain flatness. However, thereare different power levels in each channel due to differences inaccumulated channel losses at different frequencies. Variable OpticalAttenuators (VOA's) are used in the respective channels to compensatefor the losses. The VOA's require frequent adjustment to maintainrequired power levels, and if the power level in a given channel dropsbelow a minimum level, a transponder is required in the line to increasethe power level to the required level.

Thus, there is a need to be able to automatically readjust the powerlevel on a per-channel basis so that the photodetector at the opticalreceiver receives a signal with an adequate Optical Signal to NoiseRatio (OSNR) and amplitude to achieve a desired Bit Error Rate (BER),but not so high a power level that the optical receiver or theelectronics to follow are saturated.

SUMMARY OF THE INVENTION

In view of the above, it is an aspect of the invention to adjust thepower levels in an optical communication system on a per-channel basis.

It is another aspect of the invention to adjust the power levels in anoptical communication system on a per-channel basis by including in eachchannel an optical amplifier which is operated in the saturation mode.

It is yet another aspect of the inventions to adjust the power levels inan optical communication system on a per-channel basis by including ineach channel an optical amplifier, with each such amplifier receiving apredetermined pump power for operating each such amplifier in thesaturation mode.

It is still another aspect of the invention to connect Optical LineTerminals (OLT's) back-to-back at their respective pass-throughinterface channels, with each channel including an optical amplifier,with each such amplifier receiving a predetermined pump power foroperating each such amplifier in the saturation mode.

It is still yet another aspect of the invention to adjust the powerlevels in each output channel from a demultiplexer in a WDM opticalcommunication system on a per-channel basis, with each such outputchannel including an optical amplifier, with each such amplifierreceiving a predetermined pump power for operating each such amplifierin the saturation mode, with the pump power being provided from either apredetermined power per-channel pump for each amplifier, or a singleshared pump which supplies the predetermined power to each channelamplifier, wherein one or more of the pumps also are referred to as a“controller”.

It is a further aspect of the invention to adjust the power levels ineach input channel to a multiplexer in a WDM optical communicationsystem on a per-channel basis, with each such input channel including anoptical amplifier, with each such amplifier receiving a predeterminedpump power for operating each such amplifier in the saturation mode,with the pump power being provided from either a predetermined powerper-channel pump for each amplifier, or a single shared pump whichsupplies the same predetermined power to each channel amplifier.

It is yet another further aspect of the invention to maximize the numberof optical hops in an optical ring network by equalizing the outputpower level in the respective channels due operating the respectivechannel amplifiers at a predetermined power level by operating theamplifiers in the saturation mode.

It is still yet another further aspect of invention to prevent lasing inan optical ring network by operating an amplifier in each channel at apredetermined power level which can't be exceeded, such that one channelcan't rob another channel of power due to the one channel's wavelengthtraversing the loop without being dropped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art optical communication system;

FIG. 2 is a block diagram of an optical communication system accordingto the present invention;

FIG. 3 is a block diagram of a WDM optical communication systemaccording to the present invention;

FIG. 4 is a block diagram of one amplifier constituting an opticalchannel according to the present invention;

FIG. 5 is a typical graph of power-in versus power-out for the opticalamplifier 90 shown in FIG. 4;

FIG. 6 is a block diagram of a plurality of optical channels whoseoptical amplifiers receive pumping power from a shared optical pump;

FIG. 7 is a block diagram of how to couple a plurality of optical pumpsto the optical amplifiers of a plurality of optical channels; and

FIG. 8 is a block diagram of a plurality of optical nodes connected in aring configuration.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a prior art optical communication system 10in which an optical facility signal comprising multiple channels ofdifferent wavelengths is input on a single fiber 12 to an opticalamplifier 14 with flat gain which amplifies the input signal. Theamplified optical facility signal is then demultiplexed by ademultiplexer 16 into its constituent wavelengths λ1–λm, and is appliedto an Optical Cross Connect Switch (OXC) or Optical Add Drop Multiplex(OADM) 18, and then to a multiplexer 20 which multiplexes thewavelengths λ1–λm to form an optical facility signal comprising themultiple wavelengths λ1–λm which is then amplified by an opticalamplifier 22 which is identified to optical amplifier 14, which thenoutputs the amplified facility signal on output fiber 24. Wavelengthsare not shown as being added/dropped in the drawing, however, this isunderstood by those skilled in the art.

In general, even though the optical amplifiers 14 and 22 have a flatgain, the amplitudes of the individual wavelengths are often differentand require adjustment to attempt to equalize the gain of the respectivechannels. This equalization is typically accomplished using VOA's whichare inserted in the respective channels. In addition, the OXC or OADM 18introduces losses on the order of 1–5 db, which are reflected in theoutput power level of the respective channels. If the output power levelin a given channel is below a threshold level, an expensive transponderis required to raise the power level above the threshold.

FIG. 2 is a block diagram of an optical communication system accordingto the present invention, in which the output power of each channel isequalized independent of the channel wavelength and input power level.This is accomplished by including an optical amplifier in each channelwhich is controlled to operate at a predetermined power level, byoperating each optical amplifier in a saturation mode. The opticalamplifier is termed an “amplet” which is a low-cost optical amplifierusing low-cost laser pumps, in comparison to the amplifier and pumpsused for amplifying multiple wavelength facility signals.

In FIG. 2, an optical communication system 30 has an optical facilitysignal comprising multiple channels of different wavelengths input on asingle fiber 32 demultiplexed into its constituent wavelengths λ1–λn bya demultiplexer 34, which are then applied to optical amplifiers 36 a–36n, respectively in an OXC 37. Although FIG. 2 shows only one input andone output fiber, each bearing n wavelengths, in general there may bemore than one such input fiber and one such output fiber and associateddemultiplexers and multiplexers, respectively. The output power level ofeach of the optical amplifiers 36 a–36 n is at a predetermined powerlevel independent of channel wavelength and input power level due tothose amplifiers also being operated in the saturation mode. This willbe described in more detail later with respect to FIGS. 4 and 5. Therespective amplified channel wavelengths are then applied to the core 38of the OXC 37, and then the respective wavelengths are applied from thecore 38 to optical amplifiers 40 a–40 n in OXC 37. The output powerlevel of each of the optical amplifier 40 a–40 n are each at apredetermined power level due to those amplifiers also being operated inthe saturation mode. The respective amplified channel wavelengths fromOXC 37 are then multiplexed by multiplexer 42 into a multiple channelfacility signal which is output on a single fiber 44.

FIG. 3 is a block diagram of a WDM optical communication system in whichOLT's 50 and 52 are connected back-to-back to form an OADM. It is to beappreciated that there is another OADM (not shown) for optical signalflow in the opposite direction. Demultiplexer 54 and multiplexer 56 areconnected back-to-back via the channels including optical amplifiers 58,60 and 62. A multiple channel facility signal is input on a single fiber64 and is demultiplexed into its constituent wavelengths λ1–λn Rn bydemultiplexer 54. Wavelengths λ1, λ2 and λ3 are amplified by amplifiers58, 60 and 62, respectively, and are input to multiplexer 56. Wavelengthλ4 is amplified by an optical amplifier 66 and is dropped off at aclient equipment 68. Wavelength λn is dropped off at a client equipment70 without amplification. A client equipment 72 provides a wavelength λ4to multiplexer 56 via an amplifier 74, and a client equipment 76provides an unamplified signal λm to multiplexer 56. The multiplexer 56then outputs a multiple channel facility signal on a single output fiber78. The client equipment may be any one of a computer, a SONET terminal,a telephone switch, a central office switch for telephones, a digitalcross-connect switch, an end device such as a terminal, or the like.Each of the optical amplifiers 58, 60, 62, 66 and 74 are operated in thesaturation mode so that their respective output power levels are at apredetermined power level. It is to be appreciated that the channels toclient equipments 70 and 76 may also include optical amplifiers.

FIG. 4 is a block diagram of a single optical channel according to thepresent invention. An individual wavelength λx is input on a singlefiber 82 and passed by an isolator 84 to a coupler 86 which combines λxwith the light output λp from a laser pump 88. The laser pump 88 haspumping power sufficient to cause EDFA 90 to operate in the saturationmode so that its output power level is at a predetermined level. Theamplified optical wavelength λx is then passed by an isolator 92 to asingle output fiber 94.

FIG. 5 is a typical graph of power-in (Pi) versus power-out (Po) for theoptical amplifier 90 of FIG. 4. It is seen that for an input power levelof −30 db the output power level is −15 db on the steep part of thecurve, and for an input power level of −20 db the output power level is−5 db. Thus, it is seen that for a 10 db difference in input power levelthere is a 10 db difference in output power level, which difference inpower level would have to be subsequently compensated for by a VOA orthe use of a transponder in the prior art.

In contrast, it is seen that when operating on or near the flat portionof the curve the output power is substantially the same for differentinput power levels due to operating on the saturation part of the curve.For example, for an input power level of −10 db the output power levelis −4 db. Thus, it is seen for a 10 db difference between input powerlevels of −20 db and −10 db there is only a 1 db difference between theoutput power levels of −5 db and −4 db, respectively. Accordingly, itseen that if amplifiers in different channels are each operating in thesaturation mode their respective output power levels will be at apredetermined level which is substantially the same level for eachamplifier.

This is seen more clearly with respect to FIG. 6 in which four opticalchannels for four different wavelengths are shown. Each such channel isidentical to the channel 80 shown in FIG. 4, with a shared laser pump 96providing the same pumping power at λp to each of the isolators 86 a–86d, to operate each of the optical amplifiers 90 a–90 d in the saturationmode so that their respective output power levels are at substantiallythe same predetermined power level independent of channel wavelength andinput power level. It is understood that the shared pump 96 provides thesame pumping power to each of the couplers 86 a–86 d via an opticalsplitter (not shown).

FIG. 7 is a block diagram of another pump configuration in which aplurality of optical pumps are coupled to a plurality of channelamplifiers via a coupler. Channels 100 a–100 n include opticalamplifiers 102 a–102 n. Pumping power for the amplifiers 102 a–102 n areselectively provided by laser pumps 104 a–104 m via a M×N coupler 106and lines 108 a–108 n, respectively. The number of channels is equal toN, and the number of pumps is equal to M, where M and N are integers,and M is not equal to N.

For example, if there are 32 channels and each channel requires 20 MW ofpower, a 4×32 coupler can be used, with each of the 4 pumps providing160 MW of power. Thus, each pump splits power between 8 of the 32channels.

In the configuration shown in FIG. 7, one or more of the pumps 104 a–104m may be a spare pump for use in the event of another one of the pumpsbecoming inoperative.

It is understood that there may be a single pump per channel, with thepump power being the same or different for the respective amplifiers. Ifthe pump powers are different, it is understood that the respectiveamplifiers have different saturation levels.

Also, it is understood that there may be multiple shared pumps used inthe practice of the invention. For example, if there are 32 channelsthere may be 16 pumps, with 2 channels sharing a pump; or 8 pumps with 4channels sharing a pump; or 4 pumps with 8 channels sharing a pump, andso on.

FIG. 8 is a block diagram of a plurality of optical nodes 200 a–200 lconnected in a ring configuration. The respective optical nodes maycomprise OLT's, OADM's, or the like. An optical signal transmission fromone node to the next is termed a hop. If the optical nodes are OLT'sconnected back-to-back according to the prior art, up to five hops maybe made without introduction of a transponder in the lightpath. Thus ifan optical signal were transmitted from node 200 a to node 200 m, atransponder would be required at nodes 200 f and 200 k.

In contrast, according to the present invention, due to the equalizationof output power level in the respective channels in the optical ring,due to operating the respective channel amplifiers in the saturationmode, recent modeling results have shown that up to twenty-three hopsmay be made without introduction of a transponder in the lightpath.

A further advantage that is derived in such an optical ring usingamplifiers operating at a predetermined output power level in each ofthe channels, is the prevention of lasing. Since the power level outputof the amplifiers in the respective channels is constrained not to riseabove a predetermined level, a given channel's wavelength that traversesthe ring without being dropped can't rob power from another channel, dueto the respective output power levels of the amplifiers being held atthe predetermined level.

Accordingly, system cost is reduced, as fewer expensive transponders arerequired. Cost of the optical amplifiers are decreased as less gain isrequired, VOA's are not required, automatic gain control is not requiredand equalization is not required. System level costs are also decreasedas simpler software is required since no VOA control is required.Further, an inadvertent ring connection in a given channel will notcause ringing due to the amplifiers in the channel operating in thesaturation mode.

In summary, in the apparatus of the present invention each channel in anoptical communication system includes an optical amplifier whichoperates in the saturation mode such that each amplifier hassubstantially the same output power level independent of channelwavelength and input power level.

Although certain embodiments of the invention have been described andillustrated herein, it will be readily apparent to those of ordinaryskill in the art that a number of modifications and substitutions can bemade to the preferred example methods and apparatus disclosed anddescribed herein without departing from the true spirit and scope of theinvention.

1. A method of operating at least one optical node, comprising the stepsof: applying optical wavelength signals to inputs of respective ones ofa plurality of optical amplifiers; and selectively coupling output powerof a plurality of optical pumps to selected ones of the opticalamplifiers through a coupler, to cause the optical amplifiers to operatein a saturation mode, wherein a saturation level of each opticalamplifier is either substantially the same, or different, depending onthe output power of each optical pump.
 2. The method according to claim1, wherein the optical amplifiers are arranged in subsets, and theselectively coupling step selectively couples the output power of eachof the optical pumps to each optical amplifier of a corresponding one ofthe subsets.
 3. The method according to claim 1, further comprising thestep of demultiplexing an optical signal including multiple wavelengthsinto the optical wavelength signals.
 4. The method according to claim 1,wherein the optical amplifiers are arranged in subsets, and each opticalamplifier within a same subset receives at least a portion of the outputpower coupled to that subset by the coupler.
 5. An optical nodecomprising: a plurality of optical amplifiers arranged to influence apower level of corresponding channels within an optical signal; aplurality of optical pumps; and a coupler, arranged to selectivelycouple output power of the plurality of optical pumps to selected onesof the optical amplifiers, to cause the optical amplifiers to provide anoutput power level within a saturation mode, wherein the output powerlevel of each optical amplifier is either substantially the same, ordifferent, depending on the output power of each optical pump.
 6. Theoptical node according to claim 5, wherein the optical amplifiers arearranged in subsets, and the coupler couples the output power of each ofthe optical pumps to each optical amplifier of a corresponding one ofthe subsets.
 7. The optical node according to claim 5, wherein theoptical amplifiers are arranged in subsets, and each optical amplifierwithin a same subset receives at least a portion of the output powercoupled to that subset by the coupler.
 8. The optical node according toclaim 5, further comprising a spare optical pump, wherein the coupler isadapted to selectively couple an output power of the spare optical pumpto at least one of the optical amplifiers.
 9. The optical node accordingto claim 5, wherein the output power level within the saturation modeprevents lasing in an optical communication system within which theoptical node operates.
 10. A method for operating an opticalcommunication system, comprising the steps of: amplifying a power levelof each of a plurality of optical channels within at least one opticalnode of the optical communication system, through a correspondingplurality of optical amplifiers arranged to amplify respective ones ofthe optical channels; and selectively controlling the power level ofeach amplifier so that each optical amplifier operates in a saturationmode, wherein the optical amplifiers are arranged in subsets, and thecontrolling couples at least a portion of power output from each of aplurality of pump sources to each optical amplifier within a samesubset.
 11. The method according to claim 10, further comprising thestep of demultiplexing an optical signal including multiple wavelengthsinto the plurality of optical channels.
 12. The method according toclaim 10, wherein there are M pump sources and N optical amplifiers. 13.A method according to claim 10, wherein operation of each opticalamplifier in the saturation mode enables the optical channels to besuccessfully communicated through an increased number of hops relativeto a case where the optical amplifiers are not operated in thesaturation mode.
 14. An optical node, comprising: a plurality of opticalamplifiers arranged to influence a power level of each of a plurality ofoptical channels; and a controller, arranged to selectively control thepower level of each optical amplifier so that each optical amplifieroperates in a saturation mode, wherein the optical amplifiers arearranged in subsets, and the controller couples at least a portion ofpower output from each of a plurality of pump sources to each opticalamplifier within a same subset.
 15. The optical node according to claim14, wherein there are M pump sources and N optical amplifiers.
 16. Theoptical node according to claim 14, wherein operation of each opticalamplifier in the saturation mode prevents lasing in an opticalcommunication system within which the optical node operates.
 17. Theoptical node according to claim 14, wherein operation of each opticalamplifier in the saturation mode enables the optical channels to besuccessfully communicated through an increased number of hops relativeto a case where the optical amplifiers are not operated in thesaturation mode.
 18. The optical node according to claim 14, furthercomprising a core that includes at least one of an optical cross connectswitch and an optical add-drop multiplexer, wherein the core isoptically coupled to the optical channels.
 19. A method for operating anoptical communication system having at least one optical node, themethod comprising the steps of: amplifying a power level of each of aplurality of optical channels of an optical signal through correspondingones of a plurality of optical amplifiers; and providing an output ofeach of a plurality of optical pumps to each optical amplifier of acorresponding one of a plurality of subsets of the optical amplifiers,to cause the optical amplifiers of the corresponding subset to provide apredetermined output power level.